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Dear Reader,

We have had a lot to think about recent regarding polyurethane, so I think it is a good idea if we discuss the chemistry which is the basis of polyurethanes. The key chemicals for making polyurethanes are the isocyanates.

Now for those of you who have never encountered the isocyanates, I can tell you that they are rather reactive electrophiles which are often strong irritants. It is normal to make then from phosgene (carbonyl chloride) and a primary amine. They are similar to both carbon dioxide and the protein synthesis reagent (DCC, N,N’-Dicyclohexylcarbodiimide). All three have a sp carbon in an allene like system.

Using extended Huckel theory we can predict that carbon dioxide will have a charge of + 0.6 on the carbon, while diphenylcarbodiimide will have a charge of + 0.37 on the central sp carbon while the phenyl isocyanate will have a charge of + 0.46 on the central sp carbon. Here is a picture of phenyl isocyanate.

phenyl isocyanate space

Nucleophiles such as water, alcohol and amines will attack this carbon to form addition products. Here is a picture of phenyl isocyanate in which I have calculated the charges on the atoms, these are projected as colours onto the solvent accessable surface of the molecule. The more red they are then the more positive they are and the more blue then the more negative they are.

phenyl isocyanate charges

What you should be able to see is that the carbon in the isocyanate group is the most positive part of the molecule. I have a line drawing for you which will explain what happens when the molecule reacts with an alcohol, which is below.

urethane formation

In this way the isocyanate group can react with an alcohol group to form a bond between the alcohol molecule and the isocyanate molecule. The synthesis of polyurethane normally uses a diisocyanate and a long molecule which has two alcohol groups at the different ends of it. This will allow the creation of bigger and bigger molecules (polymerization) which transforms the small molecules into a very large molecule (a macromolecule).

Sex toy inspection

Dear Reader,

It has come to my attention that the Swedish chemical authority have decided to inspect some items which might be known as “adult toys” or “sex toys”. I do have a problem with this use of the word “adult” as some of the things in “adult entertainment” sector are anything but adult. To me adult means mature, sensible, reasonable and decent. The true meanings of these four words “mature, sensible, reasonable and decent” are often polar opposites of some of the things in the “adult entertainment industry”. But I think that we will leave this topic alone.

What Kemikalieinspektionen did was to consider a total of 44 items from 16 compaines, now I am not going to discuss the intended use of the items or what they are. If you feel the urge to read that then I suggest you look elsewhere. What I am going to discuss is some of the chemistry involved.

Now the Swedish body choose to consider “phthalates, short chain chlorinated paraffins, azo dyes, nickel and the metals and flame retardants that are restricted for electrical products“. While the topic and the items might be controversial I think that it was a reasonable choice to make.

Now the start of the method explains how XRF was used to screen for a range of harmful metals, it will also detect bromine. But care needs to be taken with the measurement of bromine by XRD as one of the L lines (1.48043 1.48043 1.52590 keV) for bromine are very close to the line for aluminium (K lines at 1.48670, 1.48627 and 1.55745 keV). The items which were regarded as being interesting were then sent for further examination.

The problem with the report was that it was not totally clear which analytical method was used to determine the metal or the organics in the items. What was found in one study by Gerald Fowles which is mentioned in the wonderful book “Chemistry in the Marketplace” is that the nature of the mechanical pretreatment before leaching will alter the amount of a metal which can be released from an item. The key message is that chewing a plastic children’s toy was very effective as a means of releasing the metals in them while other mechanical pretreatments tended to lock in the metals. Also sucking on toys is not that dangerous but chewing and gnawing at them does release cadmium.

He also found that the use of hydrochloric acid which contains mercury(II) chloride as a preservative also inhibited the release of cadmium from cadmium sulfide and cadmium selenide. The reason is that an even more insoluble layer of the mercury chalcogenide will form on the surface of a pigment particle thus preventing any further reaction. It is a bit like the problem of sulfuric acid and marble chips, there a layer of insoluble calcium sulfate forms on the marble chips thus preventing any further reaction from occuring.

Gerald Wilfred Albert Fowles also did some very interesting work on lead and chromium in children’s comic books when he was at Reading University in the 1970s. In Diana F. Eaton, Gerald W. A. Fowles, Michael W. Thomas, G. Brian Turnbull, Environ. Sci. Technol., 1975, 9 (8), pp 768–770 he reports on how much lead and chromium can be leached from comic books when they are leached in a simulated stomach acid.

A different approach would be to use a wet combustion of the plastic by digesting it in a Parr bomb with nitric acid. Here the plastic would be totally degraded by the oxidant (nitric acid). Then we would get all of the metal contents released from the object. It is interesting to note that simple burning can result in the loss of some semivolatile metals such as lead.

What would have been interesting to know is if the items were leached with some chemical reagent or were they digested / burnt to release the entire inventory of the metals. One interesting problem is that while when PVC and latex are heated under oxidizing conditions that they are converted totally into gases, when silicone is burnt it forms a large amount of silicone dioxide. It is possible that attempts to liberate metals from silicone objects will be hampered by the formation of silica. In the worst cases the silica may form a crust over the metals thus locking them in.

Plastic fantastic

Dear Reader,

In recent times I have shown how a lad can have fun with the unit cells of inorganic solids, but now it is time to move onto something with carbon in it. I choose to look at the solid state structure of a polymer which is a high temperature engineering polymer.

LIMMUP in the crystallographic database is Poly((4,4′-diphenylene)pyromellitimide) which was described by Y. Obata, K. Okuyama, S. Kurihara, Y. Kitano and T. Jinda in Macromolecules, 1995, 28, 1547. This is a solid which is an endless chain of atoms covalently linked to the next. Here is a picture of the unit cell.

Unit cell of the polymer

While here is a picture of five of the polymer chains.

Polymer chains

While looking for examples of the polymer chains I noticed something else, this brings me onto another subject. I hold the view that one of the first steps to maturity is the point at which a person truly accepts that things which they are not interested in, involved in or have experience in can be truly worthwhile and valid. I have to add the warning that there is some work out there which is not worthwhile and is frankly close to worthless, but I do not want to point the finger by naming names well at least not today.

I have spent much of my life working on trying to get molecules to selectively recognise metals; I used to share an office with a man (Zhixue Zhu) who worked for Howard Colquhoun on a project where he was trying to get molecules to recognise short parts of polymer chains. While it might not have been quite the sort of thing that I have done in life, I still hold the view that the work is good work which is worthwhile.

Here what Dr Zhu did was to use a pair of pyrene groups to recognise part of a short chain model of kapton (poly(4,4′-oxydiphenylene-pyromellitimide)); his tweezers recognized the pyromellitimide part of the chain. Here is a picture of the solid which he published in Chemical Communications, 2004 page 2650 together with H.M. Colquhoun, C.J. Cardin and Yu Gan.

Dr Zhu's mini tongs which grip the polymer chain

I suspect that Christine Cardin found this solid interesting as her group have done a lot of work in the past on how things like acridines bond onto DNA through pi-pi effects.

Plastics and nuclear accidents

I am aware that at Chernobyl that to stop the formation of dust that a silicone called EKOR is being used


Keeping the dust down

The events inJapanand the way in which the clean up team are dealing with the problem lead me to think about polyacrylamide. It is important from the off set to understand that polyacrylamide and acrylamide are different substances with very different properties. While polyacrylamide is made from acrylamide, good well made polyacrylamide is free of acrylamide. So it is unreasonable in my view to want to outlaw the use of polyacrylamide because the monomer (acrylamide) is toxic.

In the same way, nylon-6,6 (the stuff used for a lady’s tights) is made using cyanide. Hydrogen cyanide is reacted using a nickel catalyst with butadiene to make 1,4-dicyanobutane which is used as a feed stock for making nylon. But I do not think that wearing nylon tights (or a more macho green nylon safety belt as used by people who climb radiomasts or towers) will expose the wearer to any danger of cyanide poisoning.

Sadly some years ago in Swedena tunnel (Hallandsås Tunnel) was being dug, a rock glue which contains acrylamide was used during the construction process. Rhoca-Gil is a rock glue which contains acrylamide and a related substance (N-methylacrylamide). Sadly the monomer leaked out, it caused the deaths of fish and farmyard animals. In addition it was responsible for causing human disease in this case. (http://www.rpaltd.co.uk/documents/acrylamiderrs.pdf)

I hold the view that many monomers are nasty chemicals, and of these acrylamide is one of the nastiest ones (It is a carcinogen and a neurotoxin). But the good news is that the polymer is not able to harm you in the same way as the monomer.

In Japanat the site of the damaged nuclear plant workers have been spraying an antidust agent to stop dust becoming mobile. I have been informed by Prof Ryu Hayano (A Japanese physicist) that kuricoat (http://www.kurita.co.jp/products/kuricoat.html) is being used. One lady inJapan has kindly examined the web site for me and it appears that this product is a water based spray which gives between six and twelve months of dust suppression. Based on these details I suspect that it may be a polyacrylamide product. Such a product is soluble in water and it is known that polyacrylamide does biodegrade.

We need to ask ourselves if keeping the dust suppressed is good for the workforce, one school of thought when dealing with radioactive contamination is to immobilise and fix it. One plutonium chemist I know fromEnglandalways expressed the view that the best thing to do with plutonium contamination in the lab is to paint over it. My reasoning is the greatest threat from plutonium is the internal threat, so fixing it with paint may be the best thing to do. This is the “let sleeping dogs lie” school of thought, I would say that in many cases in chemistry that it is best to let sleeping dogs lie but it is important to take steps to make sure nobody else “wakes the dog”. For example if you find a bottle of a nasty chemical hidden behind a cupboard under the sink (I once made such a discovery) it is best not to open it without finding out how to handle it.

The paint over the plutonium is a method of giving the dog a nice comfortable bed to encourage it to go to sleep. But it is not a final solution to the problem, on the other hand if a bench is contaminated with 32P then painting and then laying a thick sheet of plastic over the bench does offer a solution as this radioisotope has a short half life.

Another use of paint to control radioactivity is the use of paint to stop radon entering a basement if your home is in a high radon area, one of the best things to do to prevent radon entering the house is to paint all the concrete surfaces in the basement and house. The paint film on the concrete will need to be renewed when the paint starts to age. Here the paint is used to provide a barrier which the radon is slow to diffuse through. The radon is given more time to decay in the walls and floor so less enters the air of the house.

But fixing radioactive dust can be an important part of the management of the contamination, by fixing radioactive dust it can provide the workers with more time to take a further action which will prevent the radioactive dust from becoming a threat to the general public and themselves.

A further reason to fix the dust in the site is to prevent non radioactive dust becoming a means for the transport of the radioactivity. If non radioactive dust enters a radioactively contaminated area then the radioactivity can become bonded onto the normal dust. This now radioactive dust could now blow around in the wind and be a threat to society.

A classic example of this is smoking and radon, I know that smoking is a horrible habit which is harmful to health. So is breathing air which contains high levels of radon, while it is not a socially horrible habit it is harmful to health. But smoking in a radon infested place is much much worse for your health, what happens is that the radioactive daughters of radon stick to the smoke particles in the cigarette smoke. These then enter the lungs and stick to your insides. Thus gluing radioactivity into your lungs. So if you live inCornwallthis is another reason to give up smoking or at least making a point of going out of the house before lighting up.

Plastic sulphur

Recently a young lady started working for me as a PhD student, she is working on the radiation processing of waste plastic.

She recently started attending a class which is about polymers, which made me think of a polymer which is unlikely to be on a course on the subject of “useful plastics”. It is important to note that while polymers / plastics are used for lots of useful things in society which you can see such as dustbin sacks, pens, clothing, rope, electrical cables, footballs, floors etc etc there are some polymers which are normally never seen outside the lab.

One of these is plastic sulphur, the method of making it is quite simple and also quite complex at the same time. When sulphur is heated it melts to form a liquid of S8 rings, on heating more it darkens as the rings open. As it heats up the S8 rings form long chains and the mixture becomes very tar like and thick. If it is suddenly cooled by pouring boiling hot sulphur into cold water the chains of sulphur do not have a chance to convert back quickly into S8 rings. The resulting sulphur is soft and rubber like. But on standing it turns back to normal sulphur, the conversion of plastic sulphur back to normal sulphur starts within an hour at room temperature so plastic sulphur can not be used as a replacement for normal rubber.

I made some plastic sulphur a few days ago, a film of this is now on youtube.

The ugly side of PVC

OK you may have read the post in which the joys of PVC are mentioned, here is the flip side of the coin. PVC is not a perfect plastic, to be honest no plastic is perfect. 

One of the problems with PVC is that in the event of a fire it forms a lot of hydrogen chloride, this is by a series of elimination reactions which form a polyenes molecule. This is not a nice clean synthesis of polyacetylene, insetad it makes a polyene which is a short length of polyacetylene which darkens the PVC and spoils it. These molecules can decompose, this can then made a range of organic products. 

Hydrogen chloride made from PVC

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